This invention relates to the formation of thin films by chemical vapor deposition (CVD) using a beam of charged particles as a source of energy to decompose a material gas into reactive species that are deposited on a substrate, thereby forming a film thereon.
Methods of forming thin films by CVD are known in which a material gas is supplied to a reaction chamber and decomposed to generate radicals that are deposited on a substrate. The most common of these known methods is plasma CVD, in which a low-ionized plasma is formed in a glow discharge. As illustrated in FIG. 1, in this known method material gas is introduced through an inlet pipe 5 into a reaction chamber 1 which has been evacuated to a pressure of several hundred millitorr via evacuation port 16. When an alternating current is applied to a high frequency electrode 17 by a high frequency power source 18, a glow discharge results which ionizes the material gas to generate a plasma. This plasma excites the material gas which then decomposes and generates radicals. The radicals are deposited on the surface of a substrate 3 which is maintained at a predetermined temperature by means of a heater 4.
However, when using CVD methods which make use of plasma as the energy source it is difficult to control the properties of the film. This difficulty arises because electrons in the plasma have a wide energy distribution. Thus, various types of radicals are generated and, therefore, the reaction(s) become varied.
Recently, another conventional method which makes use of ultraviolet light in place of plasma as the source of energy has drawn attention. This so-called photo CVD method is illustrated in FIG. 2. In the photo CVD method, material gas is introduced into a reaction chamber 1, which has been evacuated, through a material gas inlet tube 5, and ultraviolet rays 19 from a light source 20 are radiated into the material gas through an entrance window 21. At this time, the material gas is decomposed by the ultraviolet rays 19 and generates radicals. In such a photo CVD method, it is possible to select a particular wavelength of the rays from the light source. These selected photons have a particular energy level and act to generate specific radicals so that only specific reaction(s) will be promoted. Further, in a photo CVD method, it is possible to condense rays at an arbitrarily selected spot by using optical lenses or reflecting mirrors so as to improve reaction efficiency.
However, the photo CVD method has the defect that radicals generated as a result of the optical decomposition attach to and fog the ray entrance window 21, thus impeding the advancement of the reaction(s). In addition, since the light sources which have been used to date are able to provide energy of at most about 7 eV per photon, it has been impossible to use photons having an arbitrarily selected level of energy.
An object of the present invention is to overcome the above defects of the prior art, and to provide a method of forming a thin film which can ensure improved controllability over the properties of the thin film to be formed by decomposition reaction(s) of material gas.